Some of the characters in my book are based, either in part or whole, on someone I know. Do I have to let those people know they are being - for lack of a better word off the top of my head - "used" in my story? What if I change their names? If I don't (change names)? If I only use part of their name but to people who know them it's obvious who it is? (like my friend JACOB last year he made us call him DAVE and now I am the only person who still calls him DAVE and say I have a character based off him named DAVE do I have to tell him?) What might happen if I don't let them know?